What it says is what you get. If you selected Download Originals, then you're getting full-resolution images to your Mac.
What you see on your display is not "original full resolution images" because that's not how Photos works. Photos (on iOS, Mac, and in iCloud) makes "Preview" image files that incorporate all edits you may have made to your images. That means when you open an image you see the current edited version, not the unedited master image (you do have the option to view original, however).
Why does it do that? Photos does not modify your master images - those are always stored un-modified. Edits are stored as data files. This is called "non-destructive editing." Separate Previews are created so that the app doesn't have to re-render the edits every time you view the image. When you edit, the Preview is updated, the Master is unchanged. In the Photos app, if you Duplicate an image a new Preview image and a new edit data file are created. However, Photos does not duplicate the Master image (it's not necessary, just as a film photographer would not make a copy of the original negative if she was going to make multiple versions of that image).
The master photos are where they always have been when using the Photos app (and for that matter, with a slight name change, where iPhoto and Aperture photos had been stored in the past, if you used those apps). By default, those are in the Photos library in the Pictures folder on your Mac. The Photos Library is what's called a "package" - a special type of folder that requires an extra step to view the contents of that folder.
If you want to see that your photos are actually in that library, then right-click (Control-click) on the Photos library and select Show Package Contents from the menu. That will show you the internal contents of the library. You'll find a Masters folder (that name will change in macOS Catalina, but the concept remains), and copies of your master images will be neatly organized by date within that folder.
DO NOT modify the contents of the Photos library in any way. The Photos library is a database, and if you make changes to it, then the Photos app will have serious problems after that.
The way to extract full-quality copies of your images from the Photos app is to use Photos > File > Export.
